1. Field of the Invention
The invention generally relates to a power conversion circuit, and more particularly, relates to a boost converter that has an adaptive coil peak current while operating in a discontinuous switching mode.
2. Description of the Related Art
An inductor based boost converter typically has a switch controlled by a pulse width modulation (PWM) controller or a pulse frequency modulation (PFM) controller. The PWM controller uses an oscillator to generate a fixed frequency driving signal for the switch, and the driving signal's duty cycle can be varied to regulate an output voltage of the boost converter. The PWM controller can operate in a voltage mode to directly regulate the output voltage or in a current mode to indirectly regulate the output voltage by controlling a peak or an average current conducted by the switch. Both modes of operation can promote inefficiencies (e.g., switching loss or conduction loss) as load current changes.
The PFM controller does not necessarily have an oscillator and typically operates in a hysteretic mode. For example, the PFM controller is on for a burst period when the output voltage of the boost converter is less than a first level and off when the output voltage is greater than a second level. During each burst period, the PFM controller controls switching cycles for the switch. Each switching cycle involves turning on the switch until the switch conducts a fixed peak switching current and subsequently turning off the switch for a predetermined duration.
Similar to the PWM controller, the PFM controller may not operate efficiently as load current changes. For example, the fixed peak switching current is usually set for a maximum load condition. Operation under lighter load conditions can become inefficient with increased output voltage ripples and switching frequencies near an audible range. Prior methods have been proposed to improve efficiency for the PFM controller. The prior methods include operating the PFM controller in different modes in response to different load conditions.